Note: I am sadden to advise that Mr. Black has passed
away. His articles are left in tribute to his historical research. May he rest
in peace

SERRATE was a
British radar device designed to detect German night fighter radar transmissions
from their Lichtenstein aircraft interception radar (AI). At that time,
Luftwaffe fighters were causing increasing number of losses amongst RAF bombers
attacking targets in Germany and Occupied Europe. The German fighters were using
their FuG 202/212 A.I. radar to locate their targets on an established frequency
of 485-505 Mc/s.

On 6/1/42 ACM
Leigh- Mallory, AOC Fighter Command, wrote to the Under Secretary of State
saying that since the frequency of the German AI radar had now become known,
there should be a possibility that a homing radar device could be designed to
lock onto the transmissions of the German fighter radar. The suggested device
should enable his night-fighters more effectively to seek and destroy the
enemy.

There was a group
at the time called Air Interception Committee (A.I.C.) who were charged with the
responsibility of overseeing the development of devices to aid the interception
of enemy aircraft. They approved the proposal for research into ‘homer
radar’ on 3rd April 1943. As a result, Telecommunication Research Establishment
(TRE) based at Malvern were requested to investigate the possibilities. The
result was a homing radar device with the code name of SERRATE. The German radar
was in turn using transmissions from sets in the British bomber that were
switched on during the whole trip to the target and return. Later in the war,
aircrews were instructed to reduce severely the time radars were activated to
reduce the chance of detection. The RAF bombers at that time did not carry
radar that could detect the presence of the German fighters in the
vicinity.

The SERRATE system
was capable of detecting the German night fighter up to 100 miles away. The
system could indicate bearing but not range with the result that a second radar
aid had to be used in conjunction with it. This was the Air Interception Radar
AI Mk lV which itself had been in use since 1940. The AI radar could
detect an aircraft by bouncing radar signals off the mass of an aircraft at a
much shorter range than SERRATE. Homing in on the radar transmission of an enemy
aircraft was not the function of the AI Mk lV radar. The theory was that the
SERRATE would detect the enemy fighter at long range and when the range was
suitably reduced the AI Mk lV could be used for the final interception. A common
set of two Cathode Ray Tubes (CRT) was used for both radars and the operation of
a switch would convert one to another.

More advanced AI
systems were available at the time but were generally used over UK Airspace.
Since the squadron was expected to serve over continental air space it was felt
that it was more prudent to use the earlier version in case of the inevitable
destruction and capture of some aircraft. Under these requirements the
shortcomings of the AI Mk lV were ignored.

The A.I.C. then required the SERRATE Radar
system to consist of the following:

The I.F.F
interrogator was a device to differentiate friendly aircraft from enemy and was
in general use throughout Bomber Command.

The service trials
of SERRATE took place using the Beaufighter Mk. VI(f) aircraft of 141 Squadron
which was then in 12 Group of Fighter Command. A captured Ju 88 was used to
familiarise the observer/ operators with that of their forthcoming opponents.
The service trials took place between June 14/15 and September 6/7, 1943, which
resulted in a total of 179 complete sorties being undertaken. Twenty enemy
aircraft were either destroyed or damaged during these initial
sorties.

In spite of these
successes, experience quickly proved that the Beaufighter lacked both speed and
manoeuvrability to catch the German aircraft. It was calculated that one combat
resulted from every eleven sorties and that a small number of crews were much
more successful than others. For example, Wing Commander Braham achieved 9 out
of the 23 successes.

On 3rd of July 1943
a letter was addressed to the Chief of Air Staff by ‘Bert’ who was ACM Arthur
Harris; later Sir Arthur. He said that SERRATE appeared to be highly successful
and requested that Fighter Command assemble a force of 100 night-fighters fitted
with SERRATE to destroy the German night-fighters. This would encourage the
morale of Bomber Command crews and the new force should be used to support every
major raid. Portal replied that such an increase in the resources was not
practical but he was prepared to authorise the formation of three night-fighter
squadrons, which would be met by disbanding the Ranger operation flights to form
the new squadrons. The new units were to be 141,157 and 169 Squadrons; 239
Squadron is also on record as having been involved.

The Bomber Support
Group No. 100 under command of Air Cdre E.B. Addison was formed on 3rd December
1943. It had two objectives, to blind the enemy electronic eyes and ears and
secondly to destroy the defending Luftwaffe night-fighters. 141 Squadron with
its Beaufighter aircraft Mk. IV(f) was transferred to 100 Group Bomber Command
on December 3rd 1943. As the year ended it was equipped with a mixture of
Beaufighter and Mosquito II aircraft. The two other night-fighter squadrons
followed 141 into 100 Group and were fitted with the SERRATE Mk. II systems.
These too were scheduled for re-equipping with Mosquito II
aircraft.

The SERRATE Mk. II
radar was fitted in the nose of the Mosquito Mk. II in place of the more usual
nose mounted four machine guns. The RAF was scoured in the search for
Mosquito Mk II aircraft; 60 Operational Training Unit (OTU) for example had all
its Mosquito Mk II aircraft transferred to 100 Group, and replaced with other
Marks of Mosquitos.

1692 Flight had its
early history in the latter half of 1942 as the Radar Development Flight.
In June 1943 it was designated 1692 Flight dedicated to training aircrews in the
use of SERRATE. It too was transferred to 100 Group in 7th December 1943. For
training purposes Defiants and Beaufighters were fitted with radar sets of
similar characteristics to that of the FuG202/212 Lichtenstein radar sets used
by the German night-fighters. As the Group’s Mosquito units increased in numbers
the flight undertook training in other areas.

141 Squadron’s
first operation with the new group took place on December 16/17th 1943. The
Mosquito proved to be little improved upon the Beaufighter, being old and fitted
with now unreliable engines. In February – March 1944 the 100 Group replaced all
the engines of their Mosquitos with Merlin 22 versions

An experiment was
tried whereby SERRATE equipped aircraft undertook close escort of the bomber
stream. This proved to be a failure since signals from the nearby bombers
swamped the C.R.T. screens of the SERRATE fighters. It proved to be much more
productive to fly parallel to the bomber stream, 30 miles distant or to patrol
the assembly beacons in France around which German fighters would gather
awaiting instructions.

In the second
period commencing 8th December 1943 the number of SERRATE contacts began to
decrease. The widespread use of ‘WINDOW’ and SERRATE by the RAF, had
rendered the German A.I. radar almost useless. Unbeknown to the RAF, in about
June 1944, the Luftwaffe introduced gradually a new version of their radar set
the Lichtenstein SN 2 into their twin engine fighters. This change commenced
somewhere between August and October 1943.

It was February
1944 when No. 80 Wing noticed a decline in Lichtenstein transmissions, which
raised the possibility that the Germans had developed a new Lichtenstein radar
set. In a crash programme the Germans had produce 200 sets by February 1944 and
1,000 sets by May 1944. This new radar employed a frequency range of 80-85
M/cs. The use of the new German radar led to an important change in
tactics against the bomber streams and this in turn led to the Luftwaffe’s
greatest victories against Bomber Command. The use by the Luftwaffe of their new
Zahme Sau tactics and their Lichtenstein SN2 radar meant that comparatively few
of the newly equipped fighters were effective. It was some time before the
RAF fully understood the new tactics.

The Germans were
well aware that the British were homing onto their early Lichtenstein A.I.
systems. In their new radar they selected a frequency from within the band
already used by their ground plotting stations. This subterfuge proved to be
very successful since a British operator would pick up the more numerous and
powerful signals put out by the Freyas. This made the ordinary British receiver
of little practical use in trying to find and DF on to relatively weak SN2
signals. In addition, the new German radar sometimes emitted a direction finding
note thus making it easier for other German night-fighters to locate the bomber
stream.

Fitting the new
radar to the German fighters had the effect of steadily decreasing the number of
contacts with Luftwaffe night-fighters that were registered to 141 Squadron
aircraft. In addition, apparent jamming of the AI Mk. IV radar sets took place.
This effect was found to be by enemy jamming but by a defect inherent in the
design of the Mk IV AI radar. This fact was confirmed after the war by a German
historian. In an attempt to alleviate the problem, all AI Mk. IV radars were
retuned to 188 MHz from 193 MHz but this did nothing to alleviate the
situation.

For a period, the
RAF night-fighters could no longer depend upon their SERRATE radar to locate the
enemy and had to depend upon their short range AI Mk. IV
systems.

The Bomber Support
Development Unit (BSDU) was formed at Foulsham during April 1944 as a trials and
development unit for 100 Group. It carried out a wide range of functions
for 100 Group including their Radio Counter Measures (RCM) activities such as
jamming as well as supporting the SERRATE programme. It had in addition a
small production facility to make short runs of radar sets or components for use
in modifications. As an example, PERFECTOS was another type of ‘Homer’ radar set
which was said to have been developed and manufactured at BSDU
Foulsham.

The use of the
Lichtenstein SN2 led to the urgent development of SERRATE Mk IV as a method of
homing onto this new German radar. The signals were presented to the Mosquito
navigator aurally through a set of normal headphones rather than through a
cathode ray tube (CRT). The DF (direction finding) is conveyed to the operator
by coding the signal through ‘dots and dashes’ as with a Lorenz beam. This was
necessary because of the number of CRTs already in use in group aircraft for the
navigator to read, had almost reached their maximum.

When the Germans
came to design their SN-2 radar, they were aware that the British were able to
lock their A.I. radars on to their earlier design. To make their new radar more
secure, they selected a frequency within the band already used by some of their
ground plotting stations - the Freyas. This simple approach met with
considerable success because if a receiver was used to pick up the weaker SN-2
signals it would also pick up the more numerous and much stronger signals of the
FREYAS. They also made their new radar to have facilities for front and rear
scanning.

In the SERRATE Mk.
IV, using audio-frequency filtering, the FREYAS signals were almost completely
removed while the SN2 signals came through the headphones as a characteristic
high pitched note with a pronounced tremor due to the ‘Split’ transmitter
aerial. This became operational in January 1945.

The establishment
of the frequency of the SN2 became an urgent problem to solve. It was not
completely established until a Ju 88 landed in error at Woodbridge at July 13th
1944.

The Lichtenstein
SN-2 was impervious to the original British ‘WINDOW’ but when during a raid on
Stuttgart in July 1944 the lengths of RAF ‘WINDOW’ had been increased the German
night fighter operators decided their radar has developed a fault and, with
other night-fighters returned to base.

SERRATE Mk. IV and
SERRATE Mk. IV(a) became available at about the same time. The Mk. IV was a
modification of the TR 1143 receiver while the TR1430 was used as a basis of the
SERRATE Mk. IV(a). SERRATE Mk. IV appears to have been fitted to
157 and 169 Squadrons only, with 141 being omitted. Later model
Mosquito’s eventually replaced the older aircraft but for a time they were
diverted to ‘Operation Diver’ which was the destruction of Flying Bombs (FZG
76).

A SERRATE Mk. VI
appears to have been developed but little is known about its development at this
time.

A number of authors
have expressed the opinion that SERRATE was an operational failure but this is
not the view of this author. The reduction in contacts during one period was due
to the introduction of ‘WINDOW’ by the British and it had an unfortunate effect
upon SERRATE Mk. II. This was remedied relatively quickly by the introduction of
the clever design of SERRATE Mk. IV.